There are various kinds of diffraction gratings. A diffraction grating with grooves having a saw-tooth sectional shape, which is called a blazed diffraction grating, exhibits a high diffraction efficiency for a light of specific wavelength in a range from ultraviolet to visible light, and is often used in a visible light/ultraviolet monochromator and the like (see Patent Literature 1).
Known methods for producing the blazed diffraction grating include an ion beam etching for engraving a substrate by casting ion beams at a predetermined incident angle while using a photoresist as a mask and a machine cutting for forming grooves one by one with a ruling engine. The machine cutting method has a low accuracy in groove interval, and may sometimes leave an uncut chip at a processed edge part, which causes stray light. Therefore, the ion beam etching method is often used in the making of a diffraction grating that requires accuracy.
The method for producing the blazed diffraction grating by the ion beam etching method is described with reference to FIG. 1A to FIG. 1F. First, a photoresist is applied on a surface of a flat substrate 1 of quartz, glass or the like to form a photoresist layer 2 (FIG. 1A). The photoresist layer 2 is exposed under two-beam interference, and interference stripes develop on it, thereby forming a parallel-line shaped resist pattern 3 (holographic exposure) as shown in FIG. 1B. Then, using the resist pattern 3 as a mask, ion beam etching is performed with an oblique beam angle so as to achieve an intended blaze angle qB on the substrate 1 until the resist pattern 3 disappears, whereby a grating groove 4 having a saw-tooth sectional shape is formed on the substrate 1 (FIG. 1C to FIG. 1E). Thereafter, as shown in FIG. 1F, the surface of the grating groove 4 is coated with a metal film 5 of aluminum, gold or the like, and the blazed diffraction grating is completed.
In general, in the above-described etching step, the ion beam etching is performed using an etching gas that gives the etching rate for the substrate 1 higher than the etching rate for the resist pattern 3, that is, yields the selection ratio (=the etching rate for the material (for example, glass) of the substrate/the etching rate for the photoresist) larger than 1.
In this way, a master blazed diffraction grating is produced. On the grating surface of the master diffraction grating, a parting agent layer is formed, and a metal thin film is formed on the parting agent layer. Subsequently, an adhesive on a glass substrate is placed on the metal thin film. After the adhesive is hardened, the glass substrate is separated from the master diffraction grating. In this way, the metal thin film having a grating groove formed is transferred, in a reversed manner, to the glass substrate side, so that a replica diffraction grating is obtained. By using the replica diffraction grating as a master die, diffraction gratings as products are produced (see Patent Literature 1).
As the field of analysis objects of a spectrometer grows, the wavelength of the light to be diffracted by the diffraction grating has become shorter. In order to cope with the shorter diffraction wavelength, it is required to shorten the grating interval and to decrease the blaze angle.
For producing a blazed diffraction grating with a small blaze angle using the above-described ion beam etching method, the incident angle a of the ion beam needs to be large. In this case, as shown in FIG. 2, the riser surface 12 between a blaze surface 11 and an adjacent blaze surface 11 is deeply etched, and the riser surface 12 recedes under the blaze surface 11 (that is, the angle b between the blaze surface 11 and the riser surface is acute), resulting in an eaves-like shape in which the edge of the blaze surface 11 hangs over the adjacent blaze surface 11 (this is called an overhang). A replica diffraction grating cannot be produced with a master diffraction grating having such an overhang.
Further, in the case of a concave diffraction grating, if the incident angle a of the ion beam is too small, a part on the periphery remains unetched. For example, in the case of a concave diffraction grating having the radius R of curvature with the diameter D of the substrate, the edge part of the substrate is curved at an angle of q=sin−1(D/2R) from the horizontal direction, as shown in FIG. 3. Therefore, when the incident angle a of the ion beam is (90−q) degrees or larger, a part (shadow part) at which the ion beam does not reach the substrate surface remains, where the etching cannot be performed.